There are many commercial applications in which it is desirable to monitor the concentrations of components in gas streams. In particular, it is important for medical personnel to monitor the concentrations of the various components in a patient's respiratory stream to dispense the proper amount of medication and/or identify potentially hazardous conditions. This is especially important in the field of anesthesiology, where gaseous anesthetic or therapeutic agents, such as nitrous oxide, halothane, enflurane, desflurane, sevoflurane, and isoflurane, are dispensed to the patient in controlled dosages. Therefore, monitoring anesthesia may involve analyzing the respiratory stream with respect to one or more components, possibly including anesthetic or therapeutic agents, as well as other respiratory gases, such as carbon dioxide.
Spectral gas analyzers provide an indication of the presence and concentration of selected components in a gas sample based on the detected spectral composition of light transmitted through the gas sample. The gaseous components of interest can be characterized with regard to specific light absorption properties. For example, a particular gaseous component may be characterized by an absorption band at a particular wavelength or over a wavelength range. By comparing the intensity of transmitted and received light of a selected wavelength or range of wavelengths for a particular gas sample, information regarding the absorption characteristics and composition of the sample can be obtained. To monitor multiple gaseous components, some spectral gas analyzers employ multiple light sources, multiple optical filters, and/or multiple light detectors.
Spectral measurement errors may arise from variations in the optics and the optical path between the source and detector and variations in detector response. Minimizing such sources of potential error is crucial in many applications, such as monitoring respiratory and anesthetic gases.
With regard to potential optical sources of error, many spectral gas analyzers utilize lenses, mirrors, and other conventional optical elements to direct light from a source through a sample to a detector. Although it may be desirable to have a relatively long light path between the source and the detector to improve optical wavelength resolution, the light intensity can be significantly decreased due to losses along the light path. At the same time, it is desirable to have a compact instrument. To save space, the optical path may be folded by using multiple mirrors to reflect light beams between the source and the sample. Proper and consistent alignment of these lenses and mirrors relative to the source, sample, and detector is often critical for consistent and accurate spectral measurements. Keeping the surfaces of mirrors and lenses clean and free from scratches and other defects is also important.
Other sources of potential error relate to the analyzer environment. Infrared sources, such as are used in spectral gas analyzers, generally produce significant heat, which may adversely affect the stability and/or performance of other components in the analyzer, such as detectors. Further, the temperature variations cause pressure/volume changes and, therefore, variations in the absorption characteristics of the analyzed gases. It is therefore desirable to separate the infrared source from the sample and/or detector to reduce temperature fluctuations in the detector and/or sample.
In addition to eliminating sources of error in spectral gas analyzers, it is desirable to limit the number of active components, such as optical elements in the light path, thereby simplifying analyzer design, reducing costs, increasing reliability, and simplifying calibration procedures. At the same time, it is desirable to reduce analyzer size for many applications where space is limited. It is also advantageous to situate the analyzer away from the immediate vicinity of the patient to allow health care personnel to move and work with the patient, where space is typically at a premium. In this regard, analyzers should be structurally robust so as to minimize mechanical recalibration and realignment of optical components and provide for a high level of mechanical reliability.